This invention relates to polyamides containing the hexafluoroisopropylidene group ##STR1## to protective coatings and to positive photosensitive and radiation sensitive, high temperature, photoresist compositions prepared therefrom and to a method of preparing heat resistant protective coatings and relief structures of such compositions.
Positive photoresist compositions and their use are well known. Generally such photoresists compositions are prepared from alkali-soluble, phenol-formaldehyde novolak resins and light or radiation sensitive o-quinone diazides or naphthoquinone diazides. Examples of such positive photoresists are described in U.S. Pat. Nos. 3,666,473; 4,115,128; and 4,173,470.
Conventional positive novolak resists have limited temperature dimensional stability and are not well suited for modern high temperature, processes, and applications. Heat resistant, negative resists are known in the art and disclosed in U.S. Pat. Nos. 3,957,512; 4,045,223; 4,088,489; Re 30,186 and DE 3411659A1. Also heat resistant positive resists are known and disclosed in U.S. Pat. Nos. 4,093,461; 4,339,521; and 4,395,482. The advantages of positive resists over negative resists are also known in the art which include higher resolution and exposure time stability in the presence of oxygen.
In the industrial application of positive resists, the polymeric component and the photosensitizer are dissolved in an organic solvent or mixture of solvents and applied as a thin film or coating to a substrate suitable for the desired application.
The polymeric component of these photoresist formulations is soluble in aqueous alkaline solutions, but the sensitizer acts as a dissolution rate inhibitor with respect to the polymer. Upon exposure of selected areas of the coated substrate to actinic radiation, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution leaving the unexposed areas substantially intact, thus producing a positive relief pattern on the substrate.
In most applications, the exposed and developed substrate will be subjected to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, corresponds to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of photoresist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components or the manufacture of a printing plate.
The properties of a photoresist composition which are important in commercial practice include the solubility of the resist in the application solvent, the photospeed of the resist, development contrast, resist resolution, resist adhesion, dimensional stability at elevated temperature and abrasion resistance.
Photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as in projection exposure techniques where the light is passed through a series of lenses and mono-chromatic filters. Thus, high, controlled photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. Control of the photospeed is extremely important in order to produce high resolution relief patterns in microcircuitry--a photospeed too high can result in narrowing the processing conditions.
Resist resolution refers to the capability of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of a micron or so).
The ability of a resist to reproduce very small dimensions, on the order of a micron or so, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
Various attempts have been made in the prior art to produce high temperature positive resists possessing the above desired properties. For example, U.S. Pat. No. 4,093,461 discloses a heat resistant, positive resist composition comprising a quinone or naphthoquinone diazide and the polycondensation product of an aromatic dianhydride and an aromatic diamine. The properties of the positive resist of the patent are discussed in U.S. Pat. No. 4,395,482 (column 1, lines 46-64). There it is pointed out that the positive resist composition of U.S. Pat. No. 4,093,461 has limited storage life, insufficient stability to alkaline etching solutions and relatively small differences in solubility between the exposed and unexposed portion of the resist.
U.S. Pat. Nos. 4,395,482 and 4,339,521 disclose positive resist compositions containing oligomer and/or polymeric precursors of polyoxazoles in the form of a polycondensation products of aromatic and/or heterocyclic dihydroxydiamino compounds and dicarboxylic acid chlorides or esters. These references specifically disclose a poly(benzidine isophthalamide). These positive resist compositions are said to have long storage life, high heat resistance, good processability and suitability of applications in microelectronics; i.e., the preparation of finely structured patterns. They however have several disadvantages which include low solubility in the commonly used solvents and low photospeed relative to the resists of this invention.
The present invention provides an improved high temperature positive photoresist composition with improved photospeed, improved solubility in the typical organic solvents used in the preparation of photoresist and improved adhesion properties while at the same time they have good long term storage stability, in the dry and solution form, and high resolution. Additional advantages of the photoresists of this invention are higher transparency and low toxicity (the benzidine of Example 1 of U.S. Pat. No. 4,395,482 is listed as a carcinogen and it is difficult to prepare). It has been noted that the poly(benzidine isophthalamide) of the '482 patent is not gel free when prepared and; it must be filtered prior to isolation from the reaction solvent. Also a photoresist solution of this poly(benzidine isophthalamide) forms gels upon standing which is indicative of storage instability of the resist solution.